The present invention relates to a method for determining a parameter for the reproducible, production of raised contact metallizations on terminal areas of a substrate by means of depositing a metallization material in a metallization bath.
In the production of raised contact metallizations, which are also described in technical terminology as xe2x80x9cbumpsxe2x80x9d, on terminal areas of a substrate, such as, for example, a chip or a carrier substrate that serves to mount chips or other electronic components, measures have been proved necessary in practice that make possible reproducibility of the production method in order to be able to produce bumps with uniform morphology. This applies regardless of the method chosen for the bump production, in particular for methods in which the bumps are produced by a metal deposition in a metallization bath. This can be done electrolytically or by wet-chemistry.
The object of the present invention is to propose a method for determining a parameter that makes possible a simple monitoring of the processes in a metallization bath in order to be able to produce reproducible bumps in regard to their morphology.
In accordance with a first solution of the invention, a method is proposed in which, according to the invention, a test substrate having at least two terminal areas adjacent at a defined spacing is introduced into the metallization bath and the parameter is determined from the variation in an electrical quantity as a consequence of an electrical contact resulting from the buildup of contact metallizations on the adjacent terminal areas.
In the method according to the invention, the body contact between the contact metallizations that results from the continuous buildup of the contact metallizations on the terminal areas induces an electrical signal that, given a defined, that is to say known spacing between the terminal areas, makes it possible to draw a conclusion about at least the area extent of the contact metallization In an isotropic deposition process of the metallization material dissolved in the metallization bath on the terminal areas of the substrate, that is to say given a uniform xe2x80x9cgrowthxe2x80x9d of the metallization material on the surface and also in the peripheral or edge regions of the terminal area, direct conclusions are in addition possible about the shape of the raised contact metallizations.
It has proved particularly advantageous if the abovementioned parameter is determined from the variation in the electrical resistance of an electrical measuring circuit encompassing the terminal areas.
According to another aspect of the invention, a test substrate having at least one terminal area is introduced into the metallization bath and the parameter is determined from the measurement of an electrical quantity due to the mass of the contact metallization on the terminal area. In contrast to the abovementioned fist achievement, in this case, conclusions about the morphology of the individual contact metallization are not drawn from the relationship between two contact metallizations deposited on terminal areas adjacent at a defined spacing, but, on the contrary, from the metallization mass formed on the terminal area during the deposition process, which metallization mass varies the electrical properties of the unit formed from the terminal area and the metallization mass.
In this connection, it is particularly advantageous if the parameter is to determined from the value of the electrical resistance of an electrical measuring circuit connected to the terminal area. This variant of the method according to the invention is based on the fact that, in addition to the density of the metallization material, the contact metallization cross section forming on the terminal area and also the length of the contact metallization formed on the terminal area determine the value of the ohmic resistance. In the case of known electrical quantities of the terminal area, direct conclusions can be drawn therefrom about the morphology of the contact metallization, in particular in the case of an isotropic deposition process.
In order to make possible a determination of the parameter that is substantially unaffected by the measurement current, it is proposed, proceeding from the principle of the achievement on which the two achievements according to the invention are based, namely the determination of a parameter based on electrical quantities determined in an electrical measuring circuit, to close the electrical measuring circuit for determining the electrical quantity, that is to say, for example, the resistance, only intermittently, preferably periodically. As a result it is possible, for example, to substantially suppress galvanic effects as a consequence of current flow between the terminal areas or between the metallization bath and the terminal area or terminal areas on a purely wet chemical deposition process.
In practice, the influence of so-called xe2x80x9cstabilizing componentsxe2x80x9d on the deposition process or the morphology of the contact metallizations deposited in a metallization bath provided with such stabilizing components has proved appreciable in some cases. In particular, it has been found that even small deviations in the metering of such stabilizing components in a metallization bath have disproportionately large effects on the morphology of the contact metallizations deposited. This is due substantially to the fact that the stabilizing components, which are intended to prevent the decomposition of the metallization baths or an undesired deposition of the metallization material on the walls of the bath containers, have a diffusion behaviour that acts against the desired deposition on the terminal areas. In addition, the behaviour of many stabilizing components, such as, for example, lead, proves to be nonlinear with regard to the diffusion into the sure of the terminal areas, which are normally designed as aluminum pads, so that in the peripheral regions of the terminal areas a greater deposition-inhibiting effect can be detected than in the region of the surface of the terminal areas. The use of stabilizing components in a metallization bath therefore results in appreciable effects on the morphology of the contact metallizations deposited on the terminal areas. Thus, it has proved necessary to monitor the concentration of the stabilizing components in the metallization bath continuously in order to be able to produce contact me metallizations that are constant with regard to their morphology.
On the basis of the relationships cited above, it proves particularly advantageous it in the case of the method according to the invention, the concentration of a bath stabilizer added to the metallization bath is determined as parameter. In a preferred variant of the method with regard to this application case, the test substrate is provided with a plurality of terminal area arrangements that each comprise two terminal areas assigned to one another with a defined contact spacing, the contact spacings varying in the individual terminal area arrangements and the test substrate is immersed for a predetermined time interval in the metallization bath. During said time interval, the electrical resistance between the terminal areas of the respective terminal area arrangements is repeatedly measured and the stabilizer concentration is determined as a function of the maximum contact spacing of the terminal areas with short-circuit contact as a result of comparison with contact spacing reference values for known stabilizer concentrations.
Furthermore, it also proves very advantageous if the method according to the invention is used as parameter for determining the deposition rate of the metallization material in the metallization bath during the formation of the contact metallization.
In accordance with a preferred variant of the method, to determine the deposition rate, the test substrate, which is provided with at least one terminal area arrangement that comprises in each case two terminal areas assigned to one another with a defined contact spacing, is immersed in the metallization bath. The electrical resistance between the terminal areas of the terminal area arrangement is then repeatedly measured and the time to the formation of a short-circuit contact between the terminal areas is measured.
A device particularly suitable for performing the method according to the is invention comprises a test substrate having at least one terminal area arrangement that comprises two terminal areas disposed at a defined spacing from one another, the terminal areas forming short circuit poles of an electrical measuring circuit comprising a resistance measuring instrument.
To perform the method variant in which, to determine the parameter, the electrical resistance due to the mass of the contact metallization is measured, the test substrate has at least one terminal area, the terminal area forming an electrical conductor of an electrical measure circuit comprising a resistance measuring instrument. A test substrate particularly suitable for performing the method according to the invention or for forming a suitable device has at least one set of terminal area arrangements having a plurality of terminal area arrangements, the terminal area arrangements each having two terminal areas assigned to one another at a defined contact spacing and the contact spacings between terminal areas assigned to one another of various terminal area arrangements being different.
A test substrate structured in this way makes possible both the determination of the concentration of a stabilizing component contained in metallization bath and the determination of the deposition rate of the metallization material on the terminal areas.
If the test substrate comprises, in addition, a plurality of sets of terminal area arrangements, each set of terminal areas being of equal size and the various sets of terminal areas having a different size it is also possible when performing the method to detect effects that are due to the nonlinear behaviour of the diffusion of the stabilizing components into the surface of the terminal areas and that result in an anisotropic deposition on the terminal areas.
A design of the test substrate having a spatial arrangement of the terminal areas that is such that, to form the contact spacing between two terminal areas assigned to one another, the terminal areas are disposed in spaced substrate levels is suitable, in particular, for the case of a strongly anisotropic deposition of the contact material on the terminal areas.
Preferred variants of the method or preferred embodiments of the device suitable for performing the method are described in greater detail below with reference to the drawings.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.